This invention relates to sintered alumina-based ceramics and a process for producing same in which alumina grains are rendered to be grown anisotropically and which enables to achieve both high flexural strength and high fracture toughness. More specifically, this invention relates to sintered alumina-based ceramics and a process for producing same which are preferable for use as structural materials, wear resistant materials or cutting tools, each of which requires high flexural strength and fracture toughness, or suitable for use under high-temperature conditions.
The grouping of the elements is based on the international periodic table of the IUPAC version.
Alumina is a material which is thermally stable because of its high melting point and excellent in wear resistant properties and chemical stability. However, the fracture toughness of sintered alumina is generally low of around 3 MPaxc2x7m0.5 so that the main current of a structural material is non-oxide ceramics having high fracture toughness such as silicon nitride and the like now. However, it is considered that alumina is one of the essentially excellent materials in anti-oxidizing and anti-corrosive properties as compared with non-oxide ceramics since alumina is an oxide. Further, alumina can be sintered in the air, and therefor, it has the advantage of being possible to regulate its production cost low as compared with non-oxide ceramics which require the control of the sintering atmosphere. Accordingly, it is expected that the improvement of sintered alumina to have high fracture toughness will broaden the range of its applications still more.
The following processes have been disclosed for improving the fracture toughness of alumina, the lack of which has been one of the drawbacks of alumina, by increasing the resistance to crack propagation with anisotropic oxide grains introduced into sintered alumina:
(1) dispersing process of platelet-like alumina grains into alumina matrices (Japanese Patent Kokai Publication JP-A-61-256963 (1986));
(2) growing process of platelet-like alumina grains comprising the steps of admixing fluoride such as LiF, NaF, KF etc. with alumina and providing a liquid phase in the course of sintering (Japanese Patent Kokai Publication JP-A-6-87649 (1994));
(3) growing process of platelet-like alumina grains by adding a very small amount, e.g., around several hundred ppm of SiO2 to alumina (PROGRESS IN CERAMIC BASIC SCIENCE: CHALLENGE TOWARD THE 21ST CENTURY, 161-169(1996));
(4) growing process of platelet-like alumina grains by heat-treating sintered alumina after sintering (Japanese Patent Kokai Publication JP-A-7-257963 (1995));
(5) depositing process of platelet-like lanthanum xcex2-aluminate into alumina matrices (Japanese Patent Kokai Publication JP-A-63-134551 (1988));
(6) adding process of SIO2 in addition to the process (5) (Japanese Patent Kokai Publication JP-A-7-277814 (1995));
(7) depositing process of needle-like AlNbO4 grains by admixing around 40% of niobium pentoxide into alumina and sintering the resultant mixture (J.Am.Ceram.Soc.,79,(9),p. 2266-2270 (1996)); and
(8) mixing process of isotropic alumina crystal grains having a long diameter of not more than 3 xcexcm and an aspect ratio of not more than 1.5, with anisotropic alumina crystal grains having a long diameter of not less than 10 xcexcm and an aspect ratio of not less than 3 at a predetermined mixing ratio, respectively, by way of e.g., admixing alumina powder with metal oxide having an eutectic point with alumina of not more than 1600xc2x0 C. to mold the resultant admixture, and heating the molded product from the room temperature up to its sintering temperature at a rate of not less than 8xc2x0 C./min. by means of e.g., microwave heating in the course of sintering (Japanese Patent Kokai Publication JP-A-9-87008 (1997)).
In the course of eager investigations toward the present invention it has turned out that the aforementioned conventional processes involve the following problems.
In the process (1), platelet-like grains are added in order to improve the flexural strength. However, these platelet-like grains act as an inhibitor of sintering in this process wherein the platelet-like grains are premixed so that a compact sintered product cannot be obtained when large platelet-like grains are added, or the platelet-like grains are added in an increased amount for the purpose of improving the fracture toughness.
In the process (2), when alumina grains are grown in the platelet-like shape only by being effected by fluoride, that is, without admixing any disperse-phase-forming agent such as zirconia, carbides, whisker and the like (samples Nos. 8 to 12 in Example 2), fracture toughness is 4.8 MPaxc2x7m0.5 or less. Accordingly, it is difficult to say that these samples have sufficient fracture toughness.
In the process (3), the fracture toughness is 3.5 MPaxc2x7m0.5 at most, which can be obtained by the addition of 300 ppm of SiO2, and accordingly, the improving effect of this process in fracture toughness is very little. Further, the addition of only SiO2 results in producing a heterogeneous sintered product in which coarse plate like grains and fine platelet-like alumina grains are co-existing. This causes lowering both the flexural strength and fracture toughness.
In the process (4), the size of platelet-like grains is large and flexural strength value remains low to be in the range of from 380 to 530 MPa. Further, this process requires heat-treating after sintering so that production steps correspondingly increase, which are not preferable in the aspect of production cost.
In the process (5), the fracture toughness is 3.4 MPaxc2x7m0.5 at most, which indicates that the improving effect of this process in fracture toughness is little.
In the process (6), simultaneous admixing of SiO2 additionally forms alumina grains into platelet-like shape and makes possible to attain high fracture toughness. However, lanthanum-xcex2-aluminate is a material which has a lower Young""s modulus than that of alumina. Thus it would be better to grow anisotropically alumina which has high Young""s modulus for increasing the resistance to crack propagation (improving fracture toughness).
In the process (7), the flexural strength value is limited as low as 320 MPa due to the large size of needle-like AlNbO4 grains in spite that fracture toughness indicates a high value of 5.3Paxc2x7m0.5. Further, this process is not preferable in the aspect of cost, since the product contains 40% of costly niobium pentoxide.
The process (8) does not always provide products having satisfied properties, besides it is not preferable in the aspect of cost and the like since it requires the use of a special microwave heating apparatus.
It is a primary object of the present invention to provide sintered alumina-based ceramics having high flexural strength and fracture toughness free from the disadvantages encountered in the prior art. Further objects of the present invention will become apparent in the entire disclosure.
In order to provide sintered alumina-based ceramics having high flexural strength and fracture toughness, the present inventors advanced the steps of their studies as follows.
The present inventors expected improving the fracture toughness of alumina by growing anisotropically alumina grains in their sintered product because the anisotropic grains deflect the propagation path of cracks. In other words, the larger the degree of this deflection of the cracks, the larger the fracture toughness. Accordingly, the sintered product of anisotropically shaped grains exhibits a large extent of deflection as compared with the sintered product of equiaxial grains so that the sintered product having anisotropically shaped grains has high toughness.
Therefore, the present inventors studied assiduously to attain the purpose of growing alumina grains anisotropically in the course of sintering. Consequently, they found that sintering a formed (molded) mixture comprising Al2O3, VB metal oxide (the oxide of the metal of VB family) and SiO2 causes alumina grains to grow anisotropically during sintering to produce the sintered alumina-based ceramics having high flexural strength and fracture toughness, and accomplished the present invention. Namely, in accordance with the present invention, the simultaneous addition of VB metal oxide and SiO2, as compared with the addition of SiO2 only, enables to grow grains having larger aspect ratio and provide a sintered product having higher fracture toughness and higher flexural strength. Further, the present invention makes it possible to prepare an excellent sintered product in density and hardness or provide a sintered product excellent in compactness.
The present invention provides sintered alumina having high flexural strength and high fracture toughness by means of comprising VB metal oxide and SiO2 and growing alumina grains anisotropically. Further, the present invention provides a process for producing same, that is, a process comprising the steps of forming a mixture comprising Al2O3, VB metal oxide and SiO2, and sintering the formed mixture.
Alumina used in the invention may be the alumina used in the conventional sintered alumina ceramics and preferably be xcex1-alumina powder of high purity (e.g., a purity of not less than 99.99%).
VB metal oxide used in the invention may be of powders commercially available. Regarding its additional ratio, one or more kind of the VB metal oxide is added preferably of from 0.05 to 2.5 mol %, more preferably of from 0.06 to 1 mol %, most preferably of from 0.08 to 0.5 mol % relative to alumina. Short of the ratio causes insufficient compactness or densification. Excess of the ratio prevents alumina grains from growing anisotropically resulting in equiaxed grains, and consequently, sufficient improving effect of fracture toughness cannot be obtained. Accordingly, neither short nor excess of the ratio is preferable.
Examples of the VB metal oxide include Nb2O5, Ta2O5, V2O5 and the like. Among them, most preferable VB metal oxide is Nb2O5 or Ta2O5, since V2O5 volatilizes during sintering offering certain difficulty in producing the target sintered product.
SiO2 used in the invention may be powders commercially available of inorganic silicic acid and the like. Regarding its additional ratio, it is preferably 0.01 to 4 mol %, more preferably 0.03 to 2 mol %, most preferably 0.05 to 1 mol % relative to alumina. Short of the ratio prevents alumina grains from growing anisotropically to become equiaxed and consequently, sufficient improving effect of the fracture toughness cannot be obtained. On the contrary, excess of the ratio causes the growth of coarse particles, which results in lowering flexural strength. Accordingly, neither short nor excess of the ratio is preferable.
In order to produce the sintered product of the invention, the mixture of Al2O3, VB metal oxide and SiO2 may be formed and then sintered. In the course of sintering, the sintered product is possibly changed into compact and dense state and at the same time alumina grains can be grown anisotropically. The sintering temperatures may be selected among those of the conventional range and preferably range from 1320 to 1600xc2x0 C., more preferably, from 1350 to 1550xc2x0 C. and most preferably, from 1380 to 1500xc2x0 C. These preferable ranges of the sintering temperatures provide compact and dense sintered products. Short of the temperature cannot provide dense sintered products. Excess of the temperature makes alumina grains equiaxed to fail obtaining sufficient improving effect of fracture toughness to bring about coarse alumina grains and a lowered flexural strength. Accordingly, neither short nor excess of the sintering temperature is preferable.
Preferable sintering time is in the range of from 10 minutes to 5 hours, more preferably, from 1 to 4 hours, and most preferably, from 1.5 to 3 hours. Short of the sintering time fails to provide a compact sintered product or to promote the anisotoropic growth of alumina grains, which makes hard to obtain good properties. Excess of the sintering time makes alumina grains coarse and invites the lowering of flexural strength.
In the sintered product obtained in the invention, the aspect ratio (the ratio of a long axis to a short axis) of the alumina grains can be determined by mirror-polishing the sintered product, etching it by the application of thermal etching or other technique, and observing its microstructure by mean of SEM (Scanning Electron Microscope) or the like. When the microstructure is observed by SEM, it is preferred that alumina grains have not less than 2 of the mean aspect ratio. Less than 2 of the mean aspect ratio is not preferable, since it is little effective to the deflection of the cracks"" propagation path, and therefore, high fracture toughness is hard to be obtained. More preferable mean aspect ratio is from 2 to 5.
In the sintered product obtained in the present invention, percentage by area of alumina grains having a long diameter of not less than 2 xcexcm and less than 10 xcexcm and aspect ratio of not less than 2, and the percentage by area of those having a long diameter of not less than 10 xcexcm can be determined by mirror-polishing the sintered product, etching it by the application of thermal etching or other technique, observing its microstructure by mean of SEM or the like, and image-analyzing by using an apparatus for processing and analyzing images. Especially preferable sintered product, as a result of observing its microstructure by SEM and analyzing the microstructure, contains not less than 20% in terms of area of alumina grains having a long diameter of not less than 2 xcexcm and less than 10 xcexcm, especially of anisotropic alumina grains having an aspect ratio of not less than 2. A less amount of alumina grains makes hard to obtain deflection effect regarding the propagation path of cracks and fracture toughness which are especially desirable. Further, especially preferable sintered product contains less than 20% in terms of area of coarse alumina grains having a long diameter of not less than 10 xcexcm. The presence of such coarse alumina grains in an amount exceeding 20% will cause fracture resulting from coarse alumina grains and make difficult to maintain especially desirable flexural strength.
Preferable sintered product of the present invention has a flexural strength of not less than 500 MPa and a fracture toughness of not less than 5 MPaxc2x7m0.5.
The sintered product of the invention can be densified by hot pressing (HP) or hot isostatic pressing (HIP) other than normal pressure sintering which is easily operable. In order to prepare a high-density sintered product, it is preferred to prepare a sintered product having a relative density of not less than 95% followed by HIP treating the sintered product under a treating pressure of 500 to 2,000 kg/cm2 at treating temperature of 1,200 to 1,600xc2x0 C.
The sintered product of the present invention can be sintered in an oxidizing atmosphere or a non-oxidizing atmosphere of nitrogen gas, argon gas or the like, or in vacuum.
The mechanism of anisotropically growing process of alumina grains in the course of sintering process cannot be fully explained, but it is presumed that the liquid phase essentially consisting of Al2O3xe2x80x94VB metal oxide-SiO2 generated in the course of sintering probably relates to the growth of highly anisotropic alumina grains.
The hardness of the sintered product of the present invention can be enhanced by adding preferably 0.01 to 0.50 mol %, relative to Al2O3, of metal oxide such as titanium oxide (TiO2 and the like), rare earth metal oxide (Y2O3, Yb2O3 and the like) and the like in the sintered product. In this case, the anisotropy of alumina grains regarding their shape anisotropy is never deteriorated. Enhanced hardness extends the application of the sintered product of the invention and materials preferable to be used in hardness-requiring fields, especially in a wear-resistant member or a cutting tool can be provided by the present invention.